Titanium activated hafnia and/or zirconia host phosphor containing indium

ABSTRACT

An intensifying screen is disclosed containing a phosphor composition comprised of monoclinic crystals of a titanium activated hafnia phosphor host containing indium ions or indium ions in combination with neodymium ions. Indium increases speed while reducing afterglow while the further addition of neodymium allows afterglow to be eliminated. A phosphor composition in which zirconia is at least partially substituted for hafnia is also disclosed.

This is a division of Ser. No. 437,866, filed 11-16-89.

FIELD OF THE INVENTION

The invention relates to novel X-ray intensifying screens. Morespecifically, the invention relates to fluorescent screens of the typeused to absorb an image pattern of X-radiation and to emit acorresponding pattern of longer wavelength electromagnetic radiation.The invention additionally relates to certain novel phosphorcompositions and to processes for their preparation.

BACKGROUND OF THE INVENTION

A developable latent image is formed in a silver halide emulsion layerof a radiographic element when it is imagewise exposed to X-radiation.Silver halide emulsions, however, more efficiently absorb andconsequently are more responsive to longer (300 to 1500 nm) wavelengthelectromagnetic radiation than to X-radiation. Silver halide possessesnative sensitivity to both the near ultraviolet and blue regions of thespectrum and can be sensitized readily to the green, red, and infraredportions of the electromagnetic spectrum.

Consequently it is an accepted practice to employ intensifying screensin combination with silver halide radiographic elements. An intensifyingscreen contains on a support a phosphor layer that absorbs the Xradiation more efficiently than silver halide and emits to the adJacentsilver halide emulsion layer of the radiographic element longerwavelength electromagnetic radiation in an image pattern correspondingto that of the X radiation received.

The most common arrangement for X radiation exposure is to employ a dualcoated radiographic element (an element with silver halide emulsionlayers on opposite sides of a support), each emulsion layer beingmounted adjacent a separate intensifying screen. The radiographicelement is a consumable, used to record a single imagewise exposure,while the intensifying screens are used repeatedly.

If the luminescence of an intensifying screen persists after imagewiseexposure to X radiation has been terminated, there is a risk that theafterglow will expose the next radiographic element brought into contactwith the screen. Thus, the measure of a satisfactory intensifying screenis not only the intensity of the luminescence it exhibits upon exposureto X-radiation, but also the rapidity with which the luminescence decaysupon the termination of X-radiation exposure.

Of the many different phosphor compositions known, most have failed tosatisfy the practical demands of intensifying screen application forfailing to generate sufficient emission intensity upon exposure toX-radiation, for exhibiting persistent luminescence after exposure(afterglow), or a combination of both.

Phosphors employed in intensifying screens consist of a host compound,often combined with a small amount of another element that changes thehue and/or improves the efficiency of fluorescence. It has beenrecognized that useful phosphors are those in which the host compoundcontains at least one higher atomic number element to facilitateabsorption of the high energy X-radiation. For example, barium sulfate,lanthanide oxyhalides and oxysulfides, yttrium tantalate, and calciumtungstate, are widely employed phosphor host compounds.

From time to time various compounds of zirconium and hafnium have beeninvestigated as phosphors. Zirconium and hafnium are known to be atomsof essentially similar radii, 1.454Å and 1.442Å, respectively.Practically all known compounds of zirconium and hafnium correspond tothe +4 oxidation state. The chemical properties of the two elements areessentially identical.

Hale U.S. Pat. No. 2,314,699, issued Mar. 23, 1943, discloses a methodof preparing a luminescent material which comprises dispersing an oxideof an element chosen from the group consisting of beryllium, magnesium,zinc, and zirconium in a solution of a salt of an element chosen fromthe group consisting of silicon, germanium, titanium, zirconium,hafnium, and thorium, and precipitating the dioxide of the element ofthe second named group upon the oxide of the element of first namedgroup.

Leverenz U.S. Pat. No. 2,402,760, issued June 25, 1946, discloses acrystalline luminescent material represented by the general formula:

    u(BeO)v(XO.sub.2)w(YO.sub.2):xMn

where X is a metal selected from the group of metals consisting ofzirconium, titanium, hafnium, and thorium, Y is an element selected fromthe group of elements consisting of silicon and germanium, the molarratio

    u/v

being from 1/99 to 99, the molar ratio of

    u+v/w

being from 1/3 to 2. and the sum of u+v being equal to one grammolecular weight.

Zirconium and hafnium containing compounds also containing rare earthelements have also been disclosed from time to time:

Anderson U.S. Pat. No. 3,640,887, issued Feb. 8, 1972, disclosestransparent polycrystalline ceramic bodies composed of oxides ofthorium, zirconium, hafnium, and mixtures thereof with oxides of therare earth elements 58 through 71 of the Periodic Table optionallyadditionally including yttria. Anderson contains no mention ofluminescence.

Mathers U.S. Pat. No. 3,905,912, issued Sept. 16, 1975, discloses ahafnium phosphate host phosphor with an activator selected from amongterbium, praseodymium, dysprosium, thulium, and europium.

Kelsey, Jr. U.S. Pat. No. 4,006,097, issued Feb. 1, 1977, disclosesytterbium activated hafnia phosphors.

Chenot et al U.S. Pat. No. 4,068,128, issued Jan. 10, 1978, discloses asa phosphor for luminescent intensifying screens (Hf_(1-x) Zr_(x))O₂ :P₂O₅, where x is in the range of from 0 to 0.5. Eu⁺² is disclosed toenhance blue emission.

Chenot et al U.S. Pat. No. 4,112,194, issued Sept. 5, 1978, discloses asa phosphor for luminescent intensifying screens (Hf_(1-x) Zr_(x))_(3-y)A_(4y) (PO₄)₄, where x is within the range of about 0.005 to 0.5, A isselected from the group consisting of lithium, sodium, and Potassium,and y is within the range of 0.4 to 2.0. Eu⁺² is disclosed as anactivator for a green emitting phosphor.

Alexandrov et al U.S. Pat. No. 4,153,469, issued May 8, 1979, disclosesas artificial precious stones or laser elements monocrystals ofzirconium or hafnium oxide stabilized with yttrium oxide.

Klein et al U.S. Pat. No. 4,295,989, issued Oct. 20, 1981, discloses acubic yttria stabilized hafnia phosphor doped with Ce³⁺.

E. Iwase and S. Nishiyama, "Luminescence Spectra of Trivalent Rare EarthIons", Proc. Intern. Sym. Mol. Struct. Spectry., Tokyo, 1962, A-407-1 to7, report the crystal lattice constants of monoclinic hafnia andzirconia as follows:

                  TABLE I                                                         ______________________________________                                        Oxide    a-axis  b-axis      c-axis                                                                              β                                     ______________________________________                                        HfO.sub.2                                                                              5.11    5.14        5.28  99° 44'                             ZrO.sub.2                                                                              5.21    5.26        5.375 99° 55'                             ______________________________________                                    

Iwase and Nishiyama investigated hafnia and zirconia forcathodoluminescence--i.e., fluorescence response to electronbombardment. The emission characteristics of these oxides doped withtrivalent samarium, praseodymium, dysprosium, terbium, and europium ionsare reported.

It has been recognized that the inclusion of titanium as an activatorcan significantly increase the luminescence of zirconia and hafnia:

Kroger U.S. Pat. No. 2,542,336, issued Feb. 20, 1951, discloses aphosphor containing titanium as an activator and having a matrixcomposed of one or more of the oxides of zirconium, hafnium, thorium,germanium or tin, to which may be added either acid oxides or basicoxides or both.

L. H. Brixner, "Structural and Luminescent Properties of the Ln₂ HF₂ O₇-type Rare Earth Hafnates", Mat. Res. Bull., Vol. 19, pp. 143-149, 1984,describes investigations of title phosphor host compounds. Ln is definedto include not only lanthanides, but also scandium and yttrium. Afterreporting the properties of Ti⁺⁴ as an activator for rare earthhafnates, Brixner states:

We also looked at this same activator in pure HfO₂. Under 30kVp Moradiation x ray excitation, this composition also emits in a broad bandcentered around 477 nm as seen in FIG. 5. This emission has an intensityof about 1.6 times that of PAR CaWO₄ and could therefore be of interestas an x ray intensifying screen phosphor, especially in light of thesuperior absorption of HfO relative to CaWO as seen in FIG. 6.Unfortunately, the price of optical grade HFO is so prohibitive that itcannot be used in screen applications. (Emphasis added.)

J. F. Sarver, "Preparation and Luminescent Properties of Ti-ActivatedZirconia", Journal of the Electrochemical Society, Vol. 113, No. 2, Feb.1966, pp. 124-128, discloses investigations of Ti⁺⁴ activation ofzirconia. Sarver states:

At room temperature the phosphor exhibits a very rapid initialexponential decay . . . similar to CaWO₄ and MgWO₄ and some sulfidephosphors . . . . Beyond about 20 μsec, the decay rate becomes muchslower and the phosphorescence is visually detectable for a few minutes.It was found that the addition of certain mineralizers or fluxes, inparticular 1 mole % LiF, besides leading to an expected increase inparticle size during firing, also causes an increase in the intensity ofthe phosphorescence although the intensity of the fluorescence isvirtually the same . . .

Kagami et al U.S. Pat. No. 4,275,333 discloses a composition of indiumoxide and a blue, green, or red emitting phosphor.

Mikami et al U.S. Pat. No. 4,795,589 discloses a zinc silicate phosphorcontaining indium.

RELATED PATENT APPLICATIONS

RPA-1. Bryan et al U.S. Ser. No. 305,222, filed Feb. 3, 1989, titledX-RAY INTENSIFYING SCREEN, PHOSPHOR COMPOSITION, AND PROCESS OF PHOSPHORPREPARATION, commonly assigned, discloses X-ray intensifying screenscontaining a hafnia phosphor host containing zirconia in concentrationshigher than those found in optical grade hafnia. The phosphor caninclude as an activator one or a combination of titanium, rare earth,and alkali metal ions.

RPA-2. Bryan et al U.S. Ser. No. 305,310, filed Feb. 2, 1989, nowabandoned in favor of U.S. Ser. No. 393,602, filed Aug. 14, 1989,commonly assigned, titled PHOSPHOR COMPOSITION AND X-RAY INTENSIFYINGSCREEN, discloses the preparation of lithium hafnate phosphors. Thephosphor crystals are disclosed to consist essentially of oxygen and acombination of metals satisfying the relationship:

    Li.sub.2 HF.sub.1-x-y-z Zr.sub.z Sn.sub.y Ti.sub.x L.sub.w

where

L is a rare earth, where "rare earth" is therein defined to includelanthanides, yttrium, and scandium.,

w+x+y are together 0 to 0.2; and

z is up to 0.2.

RPA 3, Bryan et al U.S. Ser. No. 437,464, filed concurrently herewithand commonly assigned, each titled X-RAY INTENSIFYING SCREEN ANDPHOSPHOR COMPOSITION, discloses titanium activated zirconium oxide andhafnium oxide phosphors containing neodymium to reduce afterglow.

SUMMARY OF THE INVENTION

In one aspect, this invention is directed to a screen comprised of asupport and a fluorescent layer containing a phosphor capable ofabsorbing X-radiation and emitting longer wavelength electromagneticradiation comprised of monoclinic crystals of a titanium activatedhafnia phosphor host. The intensifying screen is characterized in thatindium is present in the monoclinic crystals in an amount sufficient toreduce afterglow.

In another aspect this invention is directed to monoclinic crystals of aphosphor host consisting essentially of at least one of zirconia andhafnia containing an amount of titanium ions sufficient to increaseluminescence intensity during exposure to stimulating radiation and anamount of indium ions sufficient to reduce the intensity of luminescencepersisting after exposure to stimulating radiation.

DESCRIPTION OF PREFERRED EMBODIMENTS

An essential and novel feature of the present invention is the discoverythat the addition of indium ions to a phosphor host consistingessentially of at least one of zirconia and hafnia containing an amountof titanium ions sufficient to increase luminescence intensity duringexposure to stimulating radiation both increases the intensity of promptemission and reduces the disadvantage of phosphorescence (alternativelyreferred to as persistent luminescence or afterglow) associated withtitania activated r zirconia and hafnia phosphors. By reducing afterglowthe invention makes titanium activated zirconia and hafnia phosphorsavailable for applications requiring prompt decay of emission upon thecessation of external stimulation.

In a specifically preferred form of the invention both indium andneodymium are incorporated into the titanium activated host phosphor. Ifneodymium is employed in the absence of indium, the specific subjectmatter of RPA-3, afterglow can be eliminated, but at the price of areduction in prompt emission. By employing indium in combination withneodymium afterglow can be entirely eliminated with indium compensatingto maintain prompt emission at or above the levels observed withoutneodymium inclusion. Thus, in combination indium and neodymium caneliminate afterglow while maintaining or exceeding the prompt emissioncapabilities of the titanium activated phosphor host exhibited in theirabsence.

Any form of radiation can be employed known to stimulate zirconia orhafnia phosphors--e.g., X-radiation, ultraviolet radiation, or cathoderays. Since the more energetic forms of radiation require a higheratomic mass for efficient absorption, it is specifically preferred toemploy a hafnia phosphor host when X-radiation is employed forstimulation.

In a specific, preferred form the invention is directed to anintensifying screen comprised of a support and a fluorescent layercontaining a phosphor capable of absorbing X-radiation and emittinglonger wavelength electromagnetic radiation comprised of monocliniccrystals of a titanium activated hafnia phosphor host. Indium or indiumin combination with neodymium provide the afterglow control anddesirable prompt emission qualities described above, making it possibleto employ a titanium activated hafnia phosphor host in an X-rayintensifying screen intended to expose silver halide radiographicelements in rapid succession. By reducing or eliminating afterglow therisk that a radiographic element mounted adjacent the X-ray intensifyingscreen will receive an imagewise exposure from the screen attributableto emission persistence from a previous X-radiation exposure is avoided.

Since the chemical similarities of zirconium and hafnium atoms preventtheir complete separation, it is appreciated that even the purestattainable forms of zirconia also contain at some residual hafnia andvice versa. The phosphor compositions of this invention are contemplatedto include as a phosphor host the full range of possible zirconia tohafnia ratios.

The hafnia phosphor hosts contemplated for use in X-ray intensifyingscreens are contemplated to satisfy the relationship:

    Hf.sub.1-z Zr.sub.z                                        (I)

where z is uP to 0.3. Optical grade hafnia, the purest form of hafniareadily commercially attainable, contains less than about 3×10⁻⁴ mole ofzirconia per mole of hafnia. Contrary to what has heretofore suggestedby the art, when the zirconia content of the hafnia phosphor host isincreased above the levels found in optical grade hafnia an increase inluminescence is observed. Preferred phosphors are therefore those inwhich z is in the range of from 4×10⁻⁴ to 0.3, most preferably from1×10⁻³ to 0.2, and optimally from 2×10⁻³ to 0.1. The practicalsignificance of this discovery is that reagent grade hafnia,commercially available with z being slightly less than 2×10⁻², can beemployed as a hafnia phosphor host.

The small amounts of other elements found in commercially availablereagent grade hafnium and zirconium source compounds are not detrimentalto intensifying screen performance. Therefore, other possible impuritiesof the phosphor host need be given no further consideration.

In the simplest form of the invention monoclinic reagent grade hafnia orzirconia can be purchased and formed into a phosphor satisfying therequirements of this invention. To form monoclinic phosphor particlescontaining a selected ratio of hafnium and zirconium, commerciallyavailable sources of zirconium and hafnium are intimately intermixed,preferably by being dissolved in a common solvent, followed bycoprecipitation. The hafnium and zirconium containing mixture is chosenso that upon firing only hafnium, zirconium, and oxygen atoms remain asresidue, any other moieties of the compounds being thermally decomposedor otherwise driven off in firing.

Common sources of hafnium and zirconium include the dioxides, the basiccarbonates, the oxychlorides, the oxynitrates, the sulfates, and thetetrachlorides. While the dioxides, the basic carbonates, and thesulfates can be used as purchased to produce phosphors, it isadvantageous for both handling and phosphor performance to convert theother sources to less soluble solids that can be fired to give themonoclinic DO₂ phosphor desired, where D represents at least one ofzirconium and hafnium. For example, treatment of aqueous hafnium andzirconium ion containing solutions with base (e.g., alkali or ammoniumhydroxide) gives a precipitate which is a mixture of hydrous hafnia andhydrous zirconia, the relative proportions of which depend upon thosepresent in the starting materials.

Other useful solids satisfying phosphor host requirements can beproduced by treating hafnium and zirconium ion containing solutions withorganic precipitating agents, since organic materials consisting ofcarbon, hydrogen, and optionally nitrogen and/or oxygen leave noobJectionable residue upon thermal decomposition.

Hafnium and zirconium can be conveniently coprecipitated ascarboxylates, such as those containing from about 2 to 20 carbon atoms.The carboxylate moieties are in one preferred form aliphaticcarboxylates containing from about 2 to 10 carbon atoms, including bothmonocarboxylates and polycarboxylates--particularly dicarboxylates, suchas oxalates, succinates, fumarates, etc. Aromatic carboxylates, such asbenzoates, phthalates, and their ring substituted homologues, are alsoconvenient to use. A particularly preferred class of carboxylates areα-hydroxycarboxylates containing from 2 to 10 carbon atoms, such asglycolates, lactates, and mandelates. Oxalic acid can be viewed aseither a dicarboxylic acid or an α-hydroxycarboxylic acid. Oxalates areparticularly preferred moieties for forming not only hafnium andzirconium compounds, but also compounds of other metals to beincorporated in forming preferred forms of the phosphor moreparticularly described below. The carboxylate moieties can form simplecarboxylates with the hafnium or zirconium or can form hafnium orzirconium carboxylate complexes including additional cations, such asalkali metal or ammonium ions.

The hafnium and zirconium carboxylates can be conveniently formed byreacting in a common solvent the acid, salt, or ester of the carboxylatewith hafnium and zirconium containing compounds in the ratios desired inthe phosphor. The hafnium and zirconium containing compounds to bereacted can be selected from among compounds such as hafniumtetrachloride, zirconium tetrachloride, hafnium oxychloride, zirconiumoxychloride, hafnium basic carbonate, zirconium basic carbonate, hafniumnitrate, zirconium nitrate, zirconium carbonate, hafnium sulfate,zirconium sulfate, and mixtures thereof.

It is also contemplated to employ hafnium and zirconium alkoxides asstarting materials. Preferred hafnium and zirconium alkoxides are thosewhich satisfy formula II:

    D(OR).sub.4                                                (II)

where

D represents at least one of zirconium and hafnium and

R represents a hydrocarbon moiety containing from about 1 to 20(preferably about 1 to 10) carbon atoms. The hydrocarbon moieties can bechosen from any convenient straight or branched chain or cyclicsaturated or unsaturated aliphatic hydrocarbon moiety--e.g., alkyl,cycloalkyl, alkenyl, or alkynyl. Alternatively the hydrocarbon moietycan be an aromatic moiety--e.g., benzyl, phenyl, tolyl, xylyl, naphthyl,etc. In a specifically preferred from R is in each instance lower alkylof from 1 to 4 carbon atoms. Hafnium and zirconium alkoxides aredisclosed in U.S. Pat. Nos. 3,297,414; 3,754,011; 4,525,468; and4,670,472, the disclosures of which are here incorporated by reference.

In addition to alkoxide and carboxylate moiety containing hafnium andzirconium compounds various chelates, such as hafnium and zirconiumβ-diketones and diaminecarboxylates can be employed. Exemplary usefulhafnium starting materials are set forth under heading III below. Allthe compounds have otherwise identical zirconium analogues. Further,although water of hydration has been omitted, it is to be understoodthat under normal ambient conditions most of the compounds exist ashydrates.

    ______________________________________                                        H-1     Hafnyl oxalate                                                                HfO(C.sub.2 O.sub.4)                                                  H-2     Hafnyl oxalic acid                                                            H.sub.2 [HfO(C.sub.2 O.sub.4).sub.2 ]                                 H-3     Dioxalatohafnium                                                              Hf(C.sub.2 O.sub.4).sub.2                                             H-4     Trioxalatohafnic acid                                                         H.sub.2 [Hf(C.sub.2 O.sub.4).sub.3 ]                                  H-5     Ammonium trioxalatohafnate                                                    (NH.sub.4).sub.2 [Hf(C.sub.2 O.sub.4).sub.3 ]                         H-6     Potassium tetraoxalatohafnate                                                 K.sub.4 [Hf(C.sub.2 O.sub.4).sub.4 ]                                  H-7     Sodium tetraoxalatohafnate                                                    Na.sub.4 [Hf(C.sub.2 O.sub.4).sub.4 ]                                 H-8     Ammonium hafnyl oxalate                                                       (NH.sub.4).sub.2 [HfO(C.sub.2 O.sub.4).sub.2 ]                        H-9     Polyoxalatopolyhafnic acids                                           H-10    Potassium hafnyl tartrate                                                     K.sub.2 [HfO(C.sub.4 H.sub.4 O.sub.6).sub.2 ]                         H-11    Tetramandelatohafnic acid                                                     H.sub.4 [Hf(O.sub.2 CCHOC.sub.6 H.sub.5).sub.4 ]                      H-12    Triglycolatohafnic acid                                                       H.sub.3 HfOH(OCH.sub.2 COO).sub.3                                     H-13    Trilactohafnic acid                                                           H.sub.3 HfOH(OCHCH.sub.3 COO).sub.3                                   H-14    Trioxodihafnium stearate                                                      Hf.sub.2 O.sub.3 (O.sub.2 C(CH.sub.2).sub.16 CH.sub.3).sub.2          H-15    Trioxodihafnium 2-ethylcaproate                                               Hf.sub.2 O.sub.3 (O.sub.2 CCHC.sub.2 H.sub.5 (CH.sub.2).sub.3                 CH.sub.3).sub.2                                                       H-16    Hafnium acetylacetonate                                                       Hf(C.sub.5 H.sub.7 O.sub.2).sub.4                                     H-17    Potassium bisnitrilotriacetohafnate                                           K.sub.2 {Hf [N(CH.sub.2 CO.sub.2).sub.3 ]}                            H-18    Hafnium ethylenediaminetetraacetic acid                                       Hf[(O.sub.2 CCH.sub.2).sub.2 NCH.sub.2 ].sub.2                        H-19    Hafnyl malonate                                                               HfO(O.sub.2 CCH.sub.2 CO.sub.2)                                       H-20    Hafnyl phthalate                                                              HfO(O.sub.2 C.sub.6 H.sub.4 CO.sub.2)                                 H-21    Hafnium tetraisopropoxide                                                     Hf(OC.sub.3 H.sub.7).sub.4                                            H-22    Hafnium tetra-t-amyloxide                                                     Hf(OC.sub.5 H.sub.11).sub.4                                           H-23    Hafnium tetra(phenoxide)                                                      Hf(OC.sub.6 H.sub.5).sub.4                                            H-24    Hafnium di(isopropoxide) bis(2-ethoxyethoxide)                                Hf(OC.sub.3 H.sub.7).sub.2 (OC.sub.2 H.sub.4 OC.sub.2 H.sub.5).sub            .2                                                                    H-25    Hafnium tetra(cyclohexoxide)                                                  Hf(OC.sub.6 H.sub.11).sub.4                                           H-26    Hafnium di(isopropoxide) bis[2-(2-n-dodecan-                                  oxyethoxy)ethoxide]                                                           Hf(OC.sub.3 H.sub.7).sub.2 (OC.sub.2 H.sub.4 OC.sub.2 H.sub.4                 OC.sub.12 H.sub.25).sub.2                                             ______________________________________                                    

Formation of the monoclinic phosphor host is achieved by heating thezirconium and/or hafnium compounds to temperatures up to and including1400° C. Higher firing temperatures can, of course, be undertaken, sincethe Phosphor possesses high thermal stability. However, it is a distinctadvantage of this invention that firing temperatures above 1400° C. arenot required. Preferred firing temperatures are in the range of fromabout 900 to 1300° C.

Firing is continued until conversion to the monoclinic phase isachieved. For maximum firing temperatures the duration of firing can beless than 1 hour. While extended firing times are possible, once thephosphor has been converted to the monoclinic crystalline form,extending the duration of firing serves no useful purpose. Generallyfiring times in the range of from 1 to 10 hours, more typically 2 to 5hours, provide full conversions of the starting materials to thephosphor composition sought.

Since the starting materials are in most instances decomposed attemperatures well below the 900° C. minimum temperature levelcontemplated for monoclinic crystal growth, it is generally convenientto heat the starting materials to a temperature above theirdecomposition temperature, but below 900° C., for an initial period topurge volatilizable materials before progressing to the highercrystallization temperatures. Typically, a preliminary heating step inthe range of from about 300 to 900° C., preferably in the range of from400 to 700° C., is undertaken.

It is also often convenient to divide firing into two or moreconsecutive steps with intermediate cooling to permit grinding and/orwashing the material. Intermediate grinding can facilitate uniformitywhile intermediate washing, typically with distilled water, reduces therisk of unwanted contaminants, such as starting material decompositionby-products.

It has been discovered that firing the phosphor in the presence of aflux of one or a combination of akali metal ions incorporates alkalimetal ion in the phosphor and dramatically increases its luminescenceintensity. A preferred class of phosphors according to the presentinvention are those that satisfy the relationship:

    DM.sub.y                                                   (IV)

or X-radiation stimulation

    Hf.sub.1-z Zr.sub.z M.sub.y                                (V)

where

M represents at least one alkali metal;

y is in the range of from 1×10⁻⁴ to 1 (preferably 0.2); and

D and z are as defined above.

Investigations have revealed that the benefits of alkali metal ioninclusion are fully realized at relatively low concentrations andincorporation of alkali metal ions in concentrations above thoserequired for maximum luminescence enhancement are not detrimental toluminescence. There is no phosphor performance basis for limiting y tovalues of 1 or less. Rather it is primarily a phosphor preparationconvenience.

Alkali metal ion inclusion in the phosphor can be convenientlyaccomplished by forming a mixture of the hafnium and/or zirconiumstarting materials discussed above and a compound capable of releasingalkali metal ions on heating. The amount of the alkali metal compoundemployed is chosen to supply alkali metal ion in a concentration inexcess of that sought to be incorporated in the phosphor. Thus, thefollowing is contemplated as a starting material relationship:

    DM.sub.m                                                   (VI)

or, specifically, for X-radiation stimulation

    Hf.sub.1-z Zr.sub.z M.sub.m                                (VI)

wherein

M represents at least one alkali metal;

m is greater than 3×10⁻² (preferably from 1×10⁻¹ to 6); and

D and z are as defined above.

The alkali metal compounds can be alkali metal analogues cf the hafniumand zirconium starting materials discussed above. Preferred alkali metalcompound starting materials include alkali metal carbonates, sulfates,oxalates, halides, hydroxides, borates, tungstates, and molybdates.Mixtures of alkali metal starting materials are contemplated,particularly when different alkali metals are being concurrentlyincorporated in the phosphor. Since in one form the hafnium and/orzirconium complexes of formula II can contain alkali metal ion, thealkali metal can wholly or in part be provided by these complexes. Aconvenient preparation approach is to employ alkali metal containinghafnium and/or zirconium complexes satisfying formula II and to increasethe alkali metal content of the starting materials by adding otheralkali metal compounds, as indicated above.

In relationships VI and VII, m can range of up to 10 or more. Most ofthe excess of alkali metal is removed during phosphor preparation. Whenan excess of alkali metal is incorporated in the phosphor, it ispreferred to divide firing into two or more sequential steps withintermediate grinding and washing to remove soluble alkali metalcompounds. This reduces the level of alkali metal compounds availablefor release during heating in a corrosive volatilized form and alsoreduces the possibility of forming less desirable secondary phases.

Investigation of alkali metal containing phosphors indicates that theyexhibit increased levels of luminescence even after extended washing hasreduced the alkali metal content to very low levels, approachingdetection limits. While it is believed that the alkali metal isincorporated into the monoclinic crystals of the phosphor, this has notbeen conclusively established. It is possible that the alkali metalcontent of the phosphor is at least partially a surface remnant of thealkali metal flux on the surface of the monoclinic crystals during theirformation during firing.

The highest levels of phosphor luminescence have been obtained byemploying lithium as an alkali metal. In a preferred form lithiumcontaining phosphors according to this invention satisfy therelationship:

    DLi.sub.y                                                  (VIII)

or, specifically, for X-radiation stimulation

    Hf.sub.1-z Zr.sub.z Li.sub.y                               (IX)

wherein

y is in the range of from 8×10⁻⁴ to 0.15 and

D and z are as defined above.

Lithium containing materials so that the hafnium, zirconium, and lithiumions present prior to heating satisfy the following relationship:

    DLi.sub.m                                                  (X)

or, specifically, for X-radiation stimulation

    Hf.sub.1-z Zr.sub.z Li.sub.m                               (XI)

wherein

m is in the range of from 4×10⁻² to 2.0 (optimally from 7×10⁻² to 1.5)and

D and z are as defined above.

When lithium is selected as the alkali metal, it has been observed that,in addition to forming a hafnia phosphor host with lithium included, asecond phase of lithium hafnate can be formed, depending upon theproportion and selection of lithium compound starting materials. Sincetitanium activated lithium hafnate lacks the luminescence intensities oftitanium and lithium activated hafnia, a preferred embodiment of theinvention, lithium starting materials and their concentrations areselected so that any overall luminescence of the two phases remainshigher than that attained in the absence of lithium. Increasing levelsof lithium carbonate employed as a starting material results first in anincrease in overall luminescence eventually followed by a decrease inoverall luminescence attributed to the formation of increasingly largerproportions of lithium hafnate. On the other hand, employing lithiumsulfate as a starting material, increasing proportions result in peakluminescence with still higher proportions of lithium sulfate resultingin a relatively constant high level of luminescence, indicating that theproportion of lithium hafnate which is formed as a second phase islimited at higher lithium sulfate concentrations in the startingmaterials.

Sodium and potassium compounds employed as starting materials in placeof lithium compounds also result in markedly increased levels ofphosphor luminescence. These alkali metal starting materials, of course,avoid any possibility of forming a lithium hafnate second phase and cantherefore be employed well above the preferred maximum concentrationlevels of lithium starting materials without any performance penalty. Onthe other hand, it has been observed that sodium and potassium ions arequite effective at lower concentrations. Therefore, when M inrelationships IV and V represents at least one of sodium and potassium,y is preferably in the range of from 6×10⁻⁴ to 7×10⁻² (optimally from8×10⁻⁴ to 7×10⁻²).

The alkali metals cesium and rubidium are also effective to increasephosphor luminescence, but to a lesser extent than lithium, sodium, andpotassium. Combinations of any and all of the alkali metals Can beemployed in preparing the phosphors of this invention. Particularlyuseful are combinations of at least two of lithium, sodium, andpotassium ions. Lithium and potassium ion combinations have producedparticularly high levels of luminescence.

The fluorescence efficiencies of the phosphors of this invention areincreased by blending with the phosphor host before firing a smallamount of a titanium activator. Titanium activation can be undertakenaccording to any conventional technique, such as any of the techniquesdescribed by Kroger, Brixnez, and Sarver, cited above and hereincorporated by reference. Hafnium, zirconium, and titanium are presentand satisfy the relationship

    DTi.sub.x                                                  (XII)

or, specifically, for X-radiation stimulation

    Hf.sub.1-z Zr.sub.z Ti.sub.x                               (XIII)

wherein

x is the range of from 3×10⁻⁴ to 1.0 (preferably 0.5 and optimally 0.25)and

D and z are as defined above.

It is possible to introduce the titanium activator by physically mixingtitania with any of the host phosphor forming materials described above.It has been discovered, however, that higher luminescence levels atlower titanium concentrations are possible when the titanium activatorin the form of a thermally decomposable compound is physically blendedwith thermally decomposable hafnium and/or zirconium compounds. Thethermally decomposable moieties of the titanium activator compounds canbe selected from among the same compound classes described in connectionwith hafnium and zirconium. Titanium carboxylates, where thecarboxylates are chosen as described above, are particularly preferredstarting materials for the incorporation of titanium.

The inclusion of titanium in the host phosphor not only greatlyincreases the total luminescence of the phosphor, but also shifts themaximum emission wavelength of the phosphor from the ultraviolet to theblue portion of the spectrum. Emissions in the blue portion of thespectrum are more useful for intensifying screen use, since the silverhalide emulsions of radiographic elements which are employed incombination with intensifying screens possess native blue sensitivityand/or can be readily spectrally sensitized to these wavelengths whilethe organic vehicle of the emulsion is transparent in the blue portionof the spectrum.

In a specifically preferred form of the invention the zirconium richhafnia phosphors include both alkali metal ion and titanium, eachintroduced as described above. In this form the phosphor satisfies therelationship:

    DM.sub.y Ti.sub.x                                          (XIV)

or, specifically, for X radiation stimulation

    Hf.sub.1-z Zr.sub.z M.sub.y Ti.sub.x                       (XV)

wherein

D, M, x, y, and z are as previously defined.

It has been surprisingly discovered that disproportionately largeenhancements of luminescence are realized when both alkali metal ion andtitanium are incorporated in the phosphor. That is, the luminescenceincreases imparted by each of the alkali metal ion and titanium alonewhen added together do not equal or even approach the magnitude of theluminescence increase imparted by a combination of alkali metal ion andtitanium employed together in the phosphor.

To reduce the persistence of luminescence following stimulation (i.e.,phosphorescence or afterglow) a small amount of indium is incorporatedin the phosphor host as a dopant. The indium can be employed in anyconvenient amount effective to reduce afterglow. The phosphor in onecompleted form consists essentially of oxygen and combined elementssatisfying one of the following relationships:

    ______________________________________                                        (XVI)             DTi.sub.x In.sub.w                                          (XVII)            Hf.sub.1-z Zr.sub.z Ti.sub.x In.sub.w                       (XVIII)           DM.sub.y Ti.sub.x In.sub.w                                  and               Hf.sub.1-z Zr.sub.z M.sub.y Ti.sub.x In.sub.w               (XIX)                                                                         ______________________________________                                    

wherein

w is in the range of from 1×10⁻⁶ to 5×10⁻³, preferably 2×10⁻⁶ to 1×10⁻³,and

D, M, w, y, and z are as previously defined.

When the concentration of indium is no greater than 1×10⁻³, indium actsnot only to reduce afterglow, but to also increase phosphor speed.

While it is possible to significantly reduce afterglow by theincorporation of indium, the addition of neodymium in combination withindium produces a further dramatic reduction in afterglow. In fact, thefurther addition of neodymium has been observed to reduce afterglow tothe limits of detection capabilities--i.e., to eliminate afterglow.Since a small reduction of prompt emission is also produced by thepresence of the neodymium in the absence of indium, it is a particularadvantage of this invention that the indium can be used to eliminate anyreduction in prompt emission attributable to the incorporation ofneodymium.

The phosphor of the invention in a specifically preferred completed formconsists essentially of oxygen and combined elements satisfying one ofthe following relationships:

    ______________________________________                                        (XX)             DTi.sub.x In.sub.w Nd.sub.v                                  (XXI)            Hf.sub.1-z Zr.sub.z Ti.sub.x In.sub.w Nd.sub.v               (XXII)           DM.sub.y Ti.sub.x In.sub.w Nd.sub.v                          and                                                                           (XXIII)          Hf.sub.1-z Zr.sub.z M.sub.y Ti.sub.x In.sub.w Nd.sub.v       ______________________________________                                    

wherein

v is up to 5×10⁻⁴, preferably from 5×10⁻⁸ to 2×10⁻⁴ and optimally from2×10⁻⁶ to 1×10⁻⁵ ; and

D, M, w, y, and z are as previously defined. When v is within the rangeof from 5×10⁻⁸ to 2× 10⁻⁴, significant afterglow reduction is observed,but at the lower end of the range afterglow is not eliminated and at thehigher end of the range significant prompt emission reduction can occur.Within the optimum range of v=2×10⁻⁶ to 1× 10⁻⁵ afterglow is sharplyreduced and prompt emission with indium also present remains near thelevels observed in the absence of both indium and neodymium.

Indium and neodymium can be introduced into the phosphor during itspreparation in any convenient conventional manner. For example, thetechniques described above for titanium incorporation also permit indiumalone or in combination with neodymium to be incorporated in thephosphor host. A preferred technique for indium and neodymiumintroduction is to mix a water soluble salt, such as indium or neodymiumnitrate, in solution or to mix a salt or oxide of indium or neodymium infinely divided form with one of the zirconium and/or hafnium startingmaterials during or prior to titanium introduction. Indium and neodymiumare distributed within the phosphor host as a dopant during firing.

The phosphors of this invention, once formed, can be employed to serveany conventional use for hafnia and/or zirconia phosphors. Aspecifically preferred application for the phosphors when z is 0.3 orless (i.e., in hafnia phosphor host formulations) is in X-rayintensifying screens. Aside from the inclusion of a phosphor satisfyingthe requirements of this invention, the intensifying screen can be ofany otherwise conventional type. In its preferred construction theintensifying screen is comprised of a support onto which is coated afluorescent layer containing the phosphor of this invention inparticulate form and a binder for the phosphor particles. The phosphorscan be used in the fluorescent layer in any conventional particle sizerange and distribution. It is generally appreciated that sharper imagesare realized with smaller mean particle sizes. Preferred mean particlesizes for the zirconium rich hafnia phosphors of this invention are inthe range of from from 0.5 μm to 40 μm, optimally from 1 μm to 20 μm.

It is, of course, recognized that the phosphor particles can be blendedwith other, conventional phosphor particles, if desired, to form anintensifying screen having optimum properties for a specificapplication. Intensifying screen constructions containing more than onephosphor containing layer are also possible, with the phosphor particlesof this invention being present in one or more of the phosphorcontaining layers.

The fluorescent layer contains sufficient binder to give the layerstructural coherence. The binders employed in the fluorescent layers canbe identical to those conventionally employed in fluorescent screens.Such binders are generally chosen from organic polymers which aretransparent to X-radiation and emitted radiation, such as sodiumo-sulfobenzaldehyde acetal of poly(vinyl alcohol); chlorosulfonatedpoly(ethylene); a mixture of macromolecular bisphenol poly(carbonates)and copolymers comprising bisphenol carbonates and poly(alkyleneoxides); aqueous ethanol soluble nylons; poly(alkyl acrylates andmethacrylates) and copolymers of alkyl acrylates and methacrylates withacrylic and methacrylic acid; poly(vinyl butyral); and poly(urethane)elastomers. These and other useful binders are disclosed in U.S. Pat.Nos. 2,502,529; 2,887,379; 3,617,285; 3,300,310; 3,300,311; and3,743,833; and in Research Disclosure, Vol. 154, February 1977, Item15444, and Vol. 182, June 1979. Particularly preferred intensifyingscreen binders are poly(urethanes), such as those commercially availableunder the trademark Estane from Goodrich Chemical Co.. the trademarkPermuthane from the Permuthane Division of ICI, Ltd., and the trademarkCargill from Cargill, Inc.

The support onto which the fluorescent layer is coated can be of anyconventional type. Most commonly, the support is a film support. Forhighest levels of image sharpness the support is typically chosen to beblack or transparent and mounted in a cassette for exposure with a blackbacking. For the highest attainable speeds a white support, such as atitania or barium sulfate loaded or coated support is employed.Specifically preferred reflective supports offering the highestattainable balance of speed and sharpness are those containingreflective microlenslets, disclosed by Roberts et al U.S. Ser. No.243,374, filed Sept. 12, 1988, titled AN X-RAY INTENSIFYING SCREENPERMITTING AN IMPROVED RELATIONSHIP OF IMAGING SPEED AND SHARPNESS,commonly assigned, now U.S. Pat. No. 4,912,337, issued Mar. 27, 1990.

Any one or combination of conventional intensifying screen features,such as overcoats, subbing layers, and the like, compatible with thefeatures described above can, of course, be employed. Both conventionalradiographic element and intensifying screen constructions are disclosedin Research Disclosure, Vol. 184, Aug. 1979, Item 18431, the disclosureof which and the Patents cited therein are here incorporated byreference. Research Disclosure is published by Kenneth MasonPublications, Ltd., Emsworth, Hampshire PO10 7DD, England.

In one specifically preferred form of the invention, illustratingintensifying screens satisfying the requirements of the inventionintended to be employed with a separate silver halide emulsion layercontaining radiographic element, the phosphor of this invention can besubstituted for any of the conventional phosphors employed in either thefront or back intensifying screens of Luckey, Roth et al U.S. Pat. No.4,710,637, the disclosure of which is here incorporated by reference.Similar modification of any of the conventional intensifying screensdisclosed in the following patents is also contemplated: DeBoer et alU.S. Pat. No. 4,637,898; Luckey, Cleare et al U.S. Pat. No. 4,259,588;and Luckey U.S. Pat. No. 4,032,471.

While the phosphors of the invention can be employed for their promptemission following exposure to X-radiation, they can also be employed asstorage phosphors--that is, for their ability to emit electromagneticradiation in a chosen wavelength range after being exposed toX-radiation and then stimulated by exposure to radiation in a thIrdspectral region. For example, the phosphors of this invention can beemployed in storage phosphor screens and systems of the type disclosedby Luckey U.S. Pat. No. 3,859,527, the disclosure of which is hereincorporated by reference. When employed in such a system the refractiveindices of the phosphor and binder are preferably approximately matched,as disclosed by DeBoer et al U.S. Pat. No. 4,637,898, also incorporatedby reference.

EXAMPLES

The invention can be better appreciated by reference to the followingspecific examples.

Examples 1-9 Phosphors Containing Varied Ratios of Hafnium and Zirconium(Hf_(1-z) Zr_(z))

The purpose of presenting these investigations is to demonstrate that,by varying the zirconium content in a hafnia host phosphor, enhancedphosphor luminescence intensity is achieved over a limited zirconiumconcentration range in which the zirconium content is higher than thatfound in optical grade hafnium sources, but still only a minorconstituent.

Hafnia phosphor samples containing varied amounts of zirconiumsubstituted for hafnium were prepared by the decomposition of theappropriate trilactohafnic and trilactozirconic acid complexes. Thecomplexes were prepared by the general method described in W. B.Blumenthal, "The Chemical Behavior of Zirconium," VanNostrand,Princeton, N.J., 1958, p 333. The varying Hf:Zr ratios are obtained byusing the appropriate mixtures of zirconium and hafnium oxychlorides inthe precipitation reactions. The oxychlorides were obtained fromTeledyne Wah Chang Albany (located at Albany, Oreg.) and used asreceived The Hf:Zr ratios in the samples were determined from theanalytical batch analyses provided by the supplier.

The preparation of trilactohafnic acid for Example 1 was carried out inthe following manner: Optical grade (Hf_(1-z) Ar_(z), z=0.000276)hafnium oxychloride (40 g) and ACS reagent lactic acid (44 g) fromEastman Kodak Company were each dissolved in about 120 ml of distilledwater. The hafnium oxychloride solution was added to the lactic acidsolution with rapid stirring to form a precipitate, and the resultingmixture was heated to 80° C. with continued stirring for about 0.5hours. The cooled mixture was filtered, and the collected solid waswashed with distilled water. After drying for 15 hours at 80° C., thesolid weighed 42 g. (for C₉ H₁₆ O₁₀ HF: theory, C=23.4%, H=3.5%; found,C=22.7 H=3.5%).

Approximately 13 g of the trilactohafnic acid was placed in a 50 mLalumina crucible, covered with an alumina lid, heated in air to 700° C.for one hour in an ashing furnace, then cooled to room temperature. Thesolid was transferred to a 20 mL alumina crucible, which was coveredwith an alumina lid. The covered 20 mL alumina crucible was placed intoa 50 mL alumina crucible, which was thereafter covered with an aluminalid. The crucible assembly was heated to 1000° C. and maintained at thattemperature for 2.5 hours before cooling to room temperature. Theresulting solid was ground with an agate mortar and pestle to give aPowder that was returned to the 20 mL alumina crucible. The 20 mLcrucible was covered with its alumina lid and then heated to 1400° C.and maintained at that temperature for 1.5 hours before cooling to roomtemperature. The resulting solid was ground with an agate mortar andpestle to give a uniform phosphor powder.

The Example 1 phosphor powder sample was made from optical grade hafniumoxychloride and contained the lowest amount of zirconium. The Example 5sample was made from reagent grade (designated by the supplier asReactor Grade Special and subsequently also referred to as R.G.S.)hafnium (Hf_(1-z) Zr_(z), z= 0.019) oxychloride. The Example 2, 3, 4A,and 4B samples were made by mixing appropriate amounts of the opticalgrade and reagent grade hafnium oxychlorides. The Example 6 to 9 sampleswere made by mixing appropriate amounts of reagent grade hafnium andzirconium oxychloride to obtain a zirconium content indicated in TableII.

The luminescence response of the phosphor powder was in this and allsubsequent Examples measured by placing the phosphor powder sample inaluminum planchets (2 mm high×24 mm diam) at a coverage of about 1.1g/cm² and exposing to X-radiation. The X-ray response was obtained usinga tungsten target X-ray source in an XRD 6™ generator. The X-ray tubewas operated at 70 kVp and 10 mA, and the X-radiation from the tube wasfiltered through 0.5 mm Cu and 1 mm Al filters before reaching thesample. The luminescent response was measured using an IP-28™photomultiplier tube at 500 V bias. The voltage from the photomultiplierwas measured with a Keithley™ high impedance electrometer and isproportional to the total light output of the sample.

The major luminescence peak of the phosphor samples was centered atabout 280 nm. This value was obtained by taking the prompt emissionspectrum of the powder using the unfiltered X-ray source describedabove. The tube was operated at 70 kVp and 30 mA. The spectrum wasacquired with an Instruments S.A. Model HR 320™ grating spectrographequipped with a Princeton Applied Research Model 1422/01™ intensifiedlinear diode array detector. The data acquisition and processing wascontrolled by a Princeton Applied Research Model 1460 OMA III™ opticalmultichannel analyzer. The spectrum was corrected for the spectralresponse of the detector spectrograph combination.

The relative luminescence intensity of the phosphor powder samples as afunction of their zirconium content is set out in Table II.

                  TABLE II                                                        ______________________________________                                        Hf.sub.1-z Zr.sub.z                                                           EXAMPLE                                                                       NO.      Zr CONTENT (z) RELATIVE INTENSITY                                    ______________________________________                                        1 (Control)                                                                              0.000276     100                                                   2          0.00040      231                                                   3         0.0010        238                                                   4A       0.01           710                                                   4B       0.01           743                                                   5         0.019         365                                                   6        0.10           350                                                   7        0.20           155                                                   8        0.30           224                                                   9 (Control)                                                                            0.50            80                                                   ______________________________________                                    

The data of Table II demonstrate that there is an enhancement in hafniaphosphor performance when the zirconium level increased over that foundin optical grade hafnium sources (represented by the Control 1). Rangesof z of from 4×10⁻⁴ (0.0004) to 0.3 are demonstrated to exhibit higherluminescence intensities than optical grade hafnia. Best results aredemonstrated when z is in the range of from 1× 10⁻³ (0.001) to 0.2,optimally in the range of from 5×10⁻³ (0.0005) to 0.1.

Examples 10-14 Preparation of Phosphors in the Presence of an AlkaliMetal Ion (DM_(m))

The purpose of presenting these investigations is to demonstrate thatthe performance of hafnia host phosphors with an elevated zirconiumlevel shown to be effective in Examples 1-9 can be further dramaticallyimproved by preparing the hafnia phosphor in the presence of an alkalimetal ion.

In each example a sample consisting of 14.72 grams of trilactohafnicacid (prepared as described in Examples 1-9 from RGS hafniumoxychloride, z=0.019) was thoroughly ground with an agate mortar andpestle with K₂ CO₃ or Li₂ CO₃ (Alfa Products; Ultra Pure grade). Themole percent of the alkali carbonate flux, based on hafnium, was chosenas indicated below in Table III. The mixtures prepared were heated asdescribed above in Examples 1 9, except for the addition of a washingstep after firing to 1000° C. This step involved washing the charge with150 mL of distilled water for 1 hour. The solid was collected and driedfor 5 minute intervals at 20, 35 and 50% power in a 500W CEM modelMDS-81™ microwave oven. The procedure described above in Examples 1-9was then completed.

X-ray diffraction analysis of the samples confirmed the presence ofmonoclinic hafnia. The presence of alkali metal ion in the phosphorpowder samples prepared in the presence of alkali carbonate flux wasconfirmed by atomic absorption analysis.

                  TABLE III                                                       ______________________________________                                        DM.sub.m                                                                      Example  M        m      Intensity (Ex. 1 = l00)                              ______________________________________                                         5       --       --     365                                                  10       K        0.2    520                                                  11       K        0.5    510                                                  12       K        2.0    545                                                  13       K        4.0    1005                                                 14       Li        0.14  1005                                                 ______________________________________                                    

A 140 to 275 percent increase in luminescence intensity relative toExample 5 is seen in the above examples containing alkali metal ion.

Referring back to Example 1, it is apparent that the hafnia phosphorsamples containing both zirconium in higher levels than found in opticalgrade hafnium sources and alkali metal ion exhibit luminescenceintensities ranging from >5 to >10 times those demonstrated by thehafnia phosphor prepared from an optical grade hafnium source.

Examples 15-18 Titanium Activated Phosphors (DTi_(x))

The purpose of presenting these investigations is to demonstrate theutility of titanium as an activator for the hafnia phosphors of thisinvention containing higher than optical grade concentrations ofzirconia. The titanium also shifts the maximum spectral emission band ofthe phosphor to visible wavelengths in the blue portion of the spectrum.

In each example a sample consisting of 14.72 grams of trilactohafnicacid (prepared as described above in Examples 1-9, z=0.019) wasthoroughly ground with varying portions of ammoniumbis(oxalato)-oxotitanium (IV), (NH₄)₂ TiO(C₂ O₄)₂ 2H₂ O, from JohnsonMatthey (99.998%). The mole percent titanium, based on hafnium, isindicated below in Table IV. The mixtures were heated and furtherexamined as in Examples 1-9.

X-ray diffraction analyses of Examples 17 and each showed traces ofunreacted TiO₂. A small amount of hafnium titanate was detected as animpurity phase in Example 18.

The relative luminescence outputs of Examples 5 and 15-18 are set out inTable IV. Not only were the luminescence outputs greatly increased inExamples 15-18, but the luminescence band maximum shifted to 475 nm,thereby providing increased emissions of visible spectrum wavelengthsmore advantageous for intensifying screen applications.

                  TABLE IV                                                        ______________________________________                                        DTi.sub.x                                                                     Example     x      Intensity (Ex. 1 = 100)                                    ______________________________________                                         5          --      365                                                       15          0.02   5330                                                       16          0.05   4000                                                       l7          0.10   2730                                                       18          0.25   l680                                                       ______________________________________                                    

From Table IV it is apparent that the inclusion of titanium in thehafnia phosphor samples containing higher than optical grade zirconiumconcentrations resulted in large increases in luminescence intensities.Thus, the titanium acted as an activator for the phosphor samples.

Examples 19-33 Preparation of Titanium Activated Phosphors in thePresence of Lithium carbonate (DTi_(x) Li_(m))

The purpose of presenting these investigations is to demonstrate thatthe performance of hafnia host phosphors with an elevated zirconiumlevel (z=0.019) and containing titanium as an activator can be furtherimproved by preparing the hafnia phosphor in the presence of an alkalimetal ion.

A sample consisting of 12.26 g of trilactohafnic acid (prepared as inExamples 1-9) was thoroughly ground with 0.1 g (5 mole percent, x= 0.05)of TiO₂ (EM Chemicals; Optipur grade) and a selected amount of Li₂ CO₃(Alfa Products; Ultrapure grade). The mixtures were processed and testedsimilarly as in Examples 10-14. In Examples 21-23 the size of thetrilactohafnic acid sample was 13.00 grams with the titania increased to0.106 g to maintain the titanium at 5 mole percent (x=0.05)

The relative intensity of the titanium activated phosphor samples as afunction of the alkali metal flux employed is given in Table V.

                  TABLE V                                                         ______________________________________                                        DTi.sub.x M.sub.m                                                             Example     m      Intensity (Ex. 1 = 100)                                    ______________________________________                                        19          0      2520                                                       20          0.01   2210                                                       21          0.02   1000                                                       22          0.06   3380                                                       23          0.10   6370                                                       24          0.10   5960                                                       25          0.20   13500                                                      26          0.20   14000                                                      27          0.40   13700                                                      28          0.50   13300                                                      29          0.50   13500                                                      30          1.0    8695                                                       31          1.5    5610                                                       32          2.0    3155                                                       33          4.0     735                                                       ______________________________________                                    

Samples in which more than 10 mole percent (m =0.20) Li₂ CO₃ was addedrevealed the presence of lithium hafnate in the X-ray Powder patterns.The amount of lithium hafnate formed in the samples increased with theLi₂ CO₃ amount. At 200 mole percent (m=4.0) Li₂ CO₃ added, lithiumhafnate is the primary phase.

From Table V it can be appreciated that values of m of from about 4×10⁻²(0.04) to 2.0 gave significantly improved results, with values of m offrom about 1×10⁻¹ (0.10) to 1.5 providing the highest luminescenceintensities observed in these comparisons.

In these comparisons it should be noted that Example 19 did not provideluminescence intensity as high as that reported in Table IV for Example16, even though both contained 5 mole percent titanium (x= 0.05) andneither was prepared in the presence of an alkali metal flux. Thisdifference is attributed to the less efficient incorporation of thetitanium activator in Example 19 resulting from employing titania ratherthan a titanium carboxylate salt as a starting material.

Examples 34-43 Preparation of Titanium Activated Phosphors in thePresence of Lithium Sulfate (DTi_(x) Li_(m))

The purpose of presenting these investigations is to demonstrate thatthe proportions of lithium hafnate formed as a second phase can becontrolled and reduced by substituting another lithium salt for lithiumcarbonate.

The same procedures were employed as in Examples 19-33, except that forLi₂ CO₃ there was substituted Li₂ SO₄ (Aldrich anhydrous: 99.99%).

The relative intensity of the titanium activated phosphor samples as afunction of the lithium sulfate flux employed is given in Table VI. InTable VI the performance data from Table V is also represented forsamples prepared using lithium carbonate at the same concentrationlevels as the lithium sulfate.

                  TABLE VI                                                        ______________________________________                                        DTi.sub.x M.sub.m                                                             Li.sub.2 CO.sub.3 Li.sub.2 SO.sub.4                                           Example                                                                              m      Intensity   Example                                                                              m      Intensity                             ______________________________________                                        20     0.01   2210        34     0.01   1545                                  21     0.02   1000        35     0.02   1545                                                            36     0.04   2105                                  22     0.06   3380        37     0.06   3605                                  23     0.10   6370        38     0.10   7645                                  24     0.10   5960                                                            25     0.20   13500       39     0.20   9115                                  26     0.20   14000                                                           28     0.50   13300       40     0.50   12400                                 30     1.0    8695        41     1.0    9820                                  32     2.0    3155        42     2.0    9330                                  33     4.0     735        43     4.0    9185                                  ______________________________________                                    

The most important advantage of employing lithium sulfate as a flux ascompared to lithium carbonate is that a reduced amount of the lithiumhafnate phase is produced. This results in significant improvements inphosphor luminescence when higher proportions of the lithium flux areemployed during phosphor formation. At lower, preferred fluxconcentrations the lithium carbonate flux yields higher luminescence.

Examples 44-47 Preparation of Phosphors in the Presence of Varied AlkaliMetal Ions

The purpose of presenting these investigations is to demonstrate thatall of the alkali metals significantly enhance phosphor luminescence.

Example 25 was repeated, except that 10 mole percent (m=0.2) of anotheralkali metal carbonate was substituted for lithium carbonate: Na₂ CO₃(0.265 g: EM Chemicals Suprapur Reagent), K₂ CO₃ (0.346 g; Alfa ProductsUltrapure grade), Rb₂ CO₃ (0.5774 g; AESAR 99.9%), or Cs₂ CO₃ (0.8146 g;AESAR 99.9%).

The luminescence intensities measured for the 1 resulting samples areset out in Table VII.

                  TABLE VII                                                       ______________________________________                                        Example   Carbonate source                                                                           Intensity (Ex. 1 = 100)                                ______________________________________                                        19        None         2520                                                   25        Li.sub.2 CO.sub.3                                                                          13500                                                  44        Na.sub.2 CO.sub.3                                                                          l0400                                                  45        K.sub.2 CO.sub.3                                                                           5400                                                   46        Rb.sub.2 CO.sub.3                                                                          3645                                                   47        Cs.sub.2 CO.sub.3                                                                          4840                                                   ______________________________________                                    

From Table VII it is apparent that all of the alkali metals areeffective to increase the luminescence of the hafnia phosphors preparedfrom sources having higher zirconium contents than found in opticalgrade sources of hafnium. From Table VII it is observed that the lowerthe atomic number alkali metals lithium, sodium, and potassium offer asignificant performance advantage over the heavier alkali metalsrubidium and cesium when equal starting concentrations are employed.

Examples 48-51 Preparation of Phosphors Using Varied Alkali MetalCompounds

The purpose of presenting these investigations is to demonstrate theutility of alkali metal compounds completed by moieties other thansulfate and carbonate.

Example 25 was repeated, except that one of the following lithiumsources was substituted for lithium carbonate: 0.2548 g Li₂ C₂ O₄ (10mole percent, m=0.2, Alfa Products reagent grade), 0.212 g LiCl (20 molepercent, m=0.2, Alfa Products anhydrous Ultrapure grade). 0.4343g LiBr(20 mole percent, m=0.2. MCB anhydrous) or 0.21 g LiOH-H₂ O (20 molepercent, m=0.2, MCB reagent).

The luminescence intensities are given in Table VIII.

                  TABLE VIII                                                      ______________________________________                                        Example   Lithium Cmpd.                                                                             Intensity (Ex. 1 = 100)                                 ______________________________________                                        19        None        2520                                                    48        Li.sub.2 C.sub.2 O.sub.4                                                                  12695                                                   49        LiCl        6730                                                    50        LiBr        9400                                                    51        LiOH:H.sub.2 O                                                                            13185                                                   ______________________________________                                    

From Table VIII it is apparent that all of the lithium compounds improvethe luminescence of the phosphor. While both lithium hydroxide andlithium oxalate produced significantly higher levels of luminescencethan the lithium halides, alkali carboxylates are clearly moreconvenient to handle than alkali hydroxides.

Examples 52-54 Enhancement of Phosphor Luminescence by a Combination ofTitanium and Alkali Metal Ion

The purpose of presenting these investigations is to demonstrate thesynergistic improvement of luminescence produced by the combination ofan alkali metal ion and the titanium activator.

Example 52

A sample consisting of 13.475 g of trilactahafnic acid (prepared asdescribed in Examples 1-9) was thoroughly ground in an agate mortar andpestle with 0.2032 g Li₂ CO₃ (10 mole percent, m= 0.2, Alfa ProductsUltrapure grade) and processed as in Examples 10-14.

Example 53

Example 15 was repeated, except that 13.475 g of trilactohafnic acid wasused with 0.44 g of TiO₂ (2 mole percent, x=0.02, EM chemicals Optipurgrade).

Example 54

Example 53 was repeated, except for the addition of 0.2032 g Li (10 molepercent, m= 0.2, Alfa Products Ultrapure grade) in the starting mixture.

The luminescence performances of Examples 5 and 52-54 are compared inTable IX.

                  TABLE IX                                                        ______________________________________                                        Example Additions       Intensity (Ex. 1 = 100)                               ______________________________________                                         5      none             365                                                  52      10 mole % Li.sub.2 CO.sub.3                                                                   1120                                                  53      2 mole % TiO.sub.2                                                                            5690                                                  54      10 mole % Li.sub.2 CO.sub.3 +                                                                 14600                                                         2 mole % TiO.sub.2                                                    ______________________________________                                    

From Table IX it is apparent that a disproportionately large increase inluminescence was realized by employing both the titanium activator andthe alkali metal ion. While each of the titanium and alkali metal aloneenhanced luminescence, a larger increase in luminescence was attainedwhen titanium and alkali metal ion were employed together than couldhave been predicted assuming the separate enhancements of luminescenceto be fully additive.

Examples 55-62 Phosphors Containing 5 Mole Percent or Less Titanium

The purpose of presenting these investigations is to demonstrate theenhancements in luminescence produced by the use as starting materialsof titanium at concentrations of 5 mole percent (x= 0.05) and less,thereby presenting a better performance definition of the lower rangesof titanium concentrations.

Potassium tetraoxalatohafnate (IV) 5 hydrate was prepared as describedin Inorg. Syn., VIII, 42 (1966) using R.G.S. hafnium oxychloride 8hydrate (z= 0.019). Upon drying at 70-90° C. for 1-16 hours in aconvection oven, the product analyzed at closer to a 3 -hydratecomposition and all subsequent use of this material was calculated asthe 3 -hydrate. Fifteen grams of the material was thoroughly ground inan agate mortar and pestle with 0.03-5 mole percent of potassiumbis(oxalato)oxotitanate (IV) 2 hydrate (Alfa Products, recrystallizedfrom ethanol). The mixtures were placed in 20 mL alumina crucibles,covered with alumina lids, and then placed in 100 mL alumina crucibles,which were covered with alumina lids. The samples were heated in air to1000° C. for 2.5 hours, then cooled to room temperature. The resultingsolids were removed from the crucibles, broken into small pieces with analumina mortar and pestle and washed by stirring in 50 mL of distilledwater. The solids were then collected and dried in a convection oven at80° C. The charges were placed in 10 mL alumina crucibles with aluminalids and heated in air to 1300° C. for 2 hours, followed by cooling toroom temperature.

The luminescence intensities of the samples are set out in Table X.

                  TABLE X                                                         ______________________________________                                        Example   Mole Percent Ti                                                                            Intensity (Ex. 1 = 100)                                ______________________________________                                         5        None          365                                                   55        0.03         5750                                                   56        0.3          6128                                                   57        1            9470                                                   58        2            10500                                                  59        3            8420                                                   60        3            9820                                                   61        4            8060                                                   62        5            9120                                                   ______________________________________                                    

From Table X it is apparent tat even at the lowest concentrations oftitanium where x=3 ×10⁻⁴, Example 55) much higher levels of luminescenceare observed than in Example 5, which lacked titanium. While some of theenhancement in luminescence as compared to Example 5 can be attributedto the presence of potassium, comparing luminescence values from TableIII, in which potassium was introduced without titanium being present,it is apparent that a part of the luminescence enhancement must beattributed to additional presence of the titanium.

Examples 63-68 Varied Levels of Zirconium in Phosphors Prepared in thePresence of Alkali Metal Ion

The purpose of presenting these investigations is to demonstrate theeffect of varied levels of zirconium in the hafnia host phosphor whenthe hafnia phosphor was prepared in the presence of alkali metal ion.

Two grades of potassium tetraoxalatohafnate (IV) 3 -hydrate wereprepared as in Example 55 from optical grade hafnium oxychloride 8hydrate and R.G.S. hafnium oxychloride 8 -hydrate. Potassiumtetraoxalatozirconate 3 -hydrate was prepared as in Example 55 fromR.G.S. zirconium oxychloride 8 -hydrate. A series of Hf_(1-z) Zr_(z) O₂samples in which z was varied from 2.76×10⁻⁴ to 6.84× 10⁻² were preparedfrom mixtures of the above precursors. The powders were combined andground in an agate mortar and pestle. The procedures of Examples 55-62were employed, with the addition of 10 mole percent K₂ CO₃ (AlfaProducts Ultrapure grade) to each sample.

Luminescence intensities as a function of zirconium levels (z) are givenin Table XI.

                  TABLE XI                                                        ______________________________________                                        Example     z         Intensity (Ex. 1 = 100)                                 ______________________________________                                        63 (Control)                                                                              2.8 × 10.sup.-4                                                                   380                                                     64          4.3 × 10.sup.-4                                                                   165                                                     65          9.6 × 10.sup.-3                                                                   770                                                     66          1.9 × 10.sup.-2                                                                   520                                                     67          4.0 × 10.sup.-2                                                                   595                                                     68          6.0 × 10.sup.-2                                                                   610                                                     ______________________________________                                         Note that Example 66 was identical to Example l0, except for employing a      different final firing temperature, and the luminescence measured was         identical.   Note that Example 66 was identical to Example 10, except for     employing a different final firing temperature, and the luminescence     measured was identical.

Table XI demonstrates that hafnia prepared from optical grade sources asin Control Example 63 yields inferior luminescence as compared tosamples in which the zirconium content z is equal to at least 1× 10⁻².Comparing Tables II and XI, it is apparent that the presence ofpotassium ion is responsible for a significant increase in luminescenceat zirconium levels equal to that in R.G.S. hafnia (z=0.019) and above.

Examples 69-72 Determinations of Alkali Metal Ion Incorporation inPhosphors Differing in Zirconium Levels

The purpose of presenting these investigations is to providequantitative determinations of alkali ion incorporation levels (y) inseveral phosphors satisfying the general relationship Hf_(1-z) Zr_(z)Ti_(x) M_(y) and having differing zirconium levels (z) satisfying therequirements of the invention.

Samples were prepared as in Examples 63-68, except for the furtheraddition of 0.2151 g of recrystallized Potassium bis(oxalato)oxotitanate(IV) 2 -hydrate (Alfa Products) to satisfy the ratio x= 0.03.

Proportions of zirconium, titanium, and potassium ion in the completedphosphor samples were determined by atomic absorption analysis andinductively coupled plasma spectrometry. The luminescence of thephosphors together with their alkali ion content observed on analysis,y(obs), are reported in Table XII. The amounts of zirconium and titaniumpresent in the starting materials, z(calc) and x(calc), are compared inTable XII to the amounts of zirconium and titanium found on analysis,z(obs) and x(obs).

                  TABLE XII                                                       ______________________________________                                        Hf.sub.1-z Zr.sub.z Ti.sub.x M.sub.y                                               Intensity                                                                     (Ex. 1 =                    x     x    y                                 Ex.  100)     z (calc)  z (obs)  (calc)                                                                              (obs)                                                                              (obs)                             ______________________________________                                        69   9820     4.3 × 10.sup.-4                                                                   4.31 × 10.sup.-4                                                                 0.03  0.022                                                                              0.022                             70   9820     9.6 × 10.sup.-4                                                                   8.79 × 10.sup.-4                                                                 0.03  0.026                                                                              0.019                             71   9820     1.9 × 10.sup.-2                                                                   1.78 × 10.sup.-2                                                                 0.03  0.031                                                                              0.025                             72   9820     4.0 × 10.sup.-2                                                                   3.87 × 10.sup.-2                                                                 0.03  0.027                                                                              0.023                             ______________________________________                                    

Although all samples exhibited similar luminescence, when acorresponding phosphor was formed from optical grade hafnium startingmaterials [z(obs)= 2.91×10⁻⁴ ], a significantly lower luminescence wasobserved.

Examples 73-105 Indium, Gallium, and Rare Earth Incorporations

The purpose of presenting these investigations is to demonstrateadvantages derived from the introduction of indium alone or incombination with neodymium to hafnia host phosphors containing titaniumas an activator.

A hydrous hafnia precursor was prepared by a conventional preparationmethod. Suitable methods are those disclosed for preparing hydrouszirconia by M. Shibagaki, K. Takahasi, and M. Matsushita, Bull. Chem.Soc. Japan, 61, 3283 (1988) and A. Benedetti, G. Fagherazzi, and F.Pinna, J. Am. Ceram. Soc., 72, 467 (1989). Samples of 1.0 mole R.G.S.hafnium cxychloride (Hf_(1-z) Zr_(z), z=0.010) from Teledyne Wah ChangAlbany and 2.1 mole of sodium hydroxide pellets from Eastman KodakCompany were each dissolved in 1.0 liter of distilled water. Thesolutions were added simultaneously to a precipitation vessel with rapidstirring. The resulting gelatinous solid was collected by vacuumfiltration and then dried using a rotary evaporator. The solid waswashed three times with 4 liters of distilled water. The collectedmaterial was then dried for 16 hours at 50° C. in a convection oven.

In each example a 0.0265 mole sample of precursor hydrous hafnia wasemployed. In all examples, except Example 73, the sample was treatedwith a measured mole percent of a gallium or indium and/or rare earthion source, either in the form of an aqueous solution or a solid. Afteraddition the ingredients were thoroughly mixed. In those instances inwhich a solution was used the samples were oven dried. Eight molepercent (based on hafnium) lithium carbonate (Aldrich, 99.997%) and 5mole percent titanium dioxide (Aldrich 99.99%) were thoroughly groundand mixed into each sample. Each sample was placed in a 10 mL aluminacrucible and covered with an alumina lid. The crucibles were heated to1000° C. and maintained at that temperature for 2.5 hours before beingallowed to cool to room temperature. The samples were each washed in 150mL of distilled water for one hour and then collected by vacuumfiltration and dried for 5 minute intervals at 20, 35 and 50 percentpower in a microwave oven. The samples were then returned to their 10 mLcrucibles in ambient air, covered, and heated to 1300° C. and maintainedat that temperature for 1.5 hours before being allowed to cool to roomtemperature. The resulting powders were ground to give uniform phosphorpowders.

To provide the best possible control for purposes of comparison, severalExample 73 control samples were prepared. During each firing of galliumor indium and/or rare earth doped phosphor samples one of the Example 73control samples was present so that the control would experience exactlythe same firing conditions as the rare earth doped phosphor beinginvestigated. In the tables below, when the gallium or indium and/orrare earth doped phosphors reported were not all fired simultaneously,the relative intensity and afterglow for the control Example 73 was anaverage of the controls fired with that group of phosphors. Relativeintensities of the control samples ranged from 13,460 to 14,520 (Ex.1=100). To facilitate comparisons, the relative intensity and relativeafterglow characteristics of the control sample (or control sampleaverage) reported in the tables below were each set at 100.

The afterglow characteristics of each phosphor sample were determined byplacing the sample in a chamber and exposing it to X-radiation from atungsten target, beryllium window tube operated at 70kVp and 10 mA,filtered with 0.5 mm Cu and 1 mm Al. The phosphor samples were preparedby placing the phosphor powder in aluminum planchets (2 mm deep×24 mmdiameter) at a coverage of about 1.1 g/cm². The emitted light wasdetected by a photomultiplier tube, the output current of which wasmeasured by a voltmeter across a load resistor. The voltmeter readingserved as an input to an x-y recorder which plotted the variation ofvoltage as a function of time. Constant irradiation by X-rays of eachsample produced a steady state reading on the voltmeter, which wasadjusted to 100% on the x-y recorder. The X-radiation was then shut off,and the decay of the light emitted by each sample was monitored. Theelapsed time required for the signal to decay to 1% of its steady statevalue was then read off the x-y plot. The minimum elapsed timemeasurable by this technique was 0.35 second. References to "afterglowat or below detection limits" are intended to indicate an elapsed timeto reach 1% of steady state emission levels of 0.35 second or less. Asemployed herein, statements of afterglow elimination indicate afterglowat or below detection limits. To facilitate comparison, control Example73, lacking gallium, indium, or rare earth, was assigned a relativeafterglow value of 100 percent, and the successive examples wereassigned a relative afterglow value based on its relationship to controlExample 73.

Comparative Examples 73-85 Neodymium Addition

The purpose of presenting these investigations is to demonstrate thathost phosphors satisfying the requirements of this invention havingtheir intensity of prompt emission increased by titanium incorporationhave their prompt emission reduced by the incorporation of neOdymium inamounts sufficient to reduce afterglow. Neodymium ions were provided inthe form of aqueous solutions of Nd(NO₃)₃ 5H₂ O (Alfa, 99.9%) inExamples 74-81 inclusive with the solid nitrate being used in Examples82-85 inclusive.

The relative luminescence outputs and afterglow values of the neodymiumdoped samples and the control lacking neodymium addition are set out inTable XIII.

                  TABLE XIII                                                      ______________________________________                                        DTi.sub.x Nd.sub.v                                                                                    Relative Relative                                     Example   v             Intensity                                                                              Afterglow                                    ______________________________________                                        73a (Control)                                                                           0.00          100      100                                          74 (Comp. Ex.)                                                                           1.25 × 10.sup.-7                                                                     98       63                                           75 (Comp. Ex.)                                                                          2.5 × 10.sup.-7                                                                       94       43                                           76 (Comp. Ex.)                                                                          5.0 × 10.sup.-7                                                                       97       55                                           77 (Comp. Ex.)                                                                          1.0 × 10.sup.-6                                                                       93       23                                           78 (Comp. Ex.)                                                                          5.0 × 10.sup.-6                                                                       90       1.4                                          79 (Comp. Ex.)                                                                          1.0 × 10.sup.-5                                                                       86       *                                            80 (Comp. Ex.)                                                                          2.5 × 10.sup.-5                                                                       82       *                                            81 (Comp. Ex.)                                                                          1.0 × 10.sup.-4                                                                       70       *                                            82 (Comp. Ex.)                                                                          2.0 × 10.sup.-4                                                                       63       *                                            83 (Comp. Ex.)                                                                          5.0 × 10.sup.-4                                                                       61       *                                            84 (Comp. Ex.)                                                                          1.0 × 10.sup.-3                                                                       57       *                                            85 (Comp. Ex.)                                                                          2.0 × 10.sup.-3                                                                       50       *                                            ______________________________________                                         *Afterglow at or below detection limit with relative afterglow not            exceeding 1                                                              

From Table XIII it is aPparent that in all instances the prompt emissionrelative intensity of the phosphor was reduced by the incorporation ofneodymium.

Examples 86-93 Indium Addition

The purpose of presenting these investigations is to demonstrate thathost phosphors satisfying the requirements of this invention havingtheir intensity of prompt emission increased by titanium incorporationhave their intensity of prompt emission further increased and theirafterglow significantly reduced by the incorporation of indium. Indiumions were provided in the form of aqueous solutions of indium nitrate inExamples 86 and 87 or solid indium oxide in Examples 88-93 inclusive.

The relative luminescence outputs and afterglow values of the neodymiumdoped samples and the control lacking rare earth addition are set out inTable XIV.

                  TABLE XIV                                                       ______________________________________                                        DTi.sub.x In.sub.w                                                                                    Relative Relative                                     Example    w            Intensity                                                                              Afterglow                                    ______________________________________                                        73b (Control)                                                                            0.00         100      100                                          86         2.5 × 10.sup.-6                                                                      106      89                                           87         5.0 × 10.sup.-5                                                                      113      68                                           88         1.0 × 10.sup.-4                                                                      118      69                                           89         2.5 × 10.sup.-4                                                                      118      64                                           90         5.0 × 10.sup.-4                                                                      105      67                                           91         1.0 × 10.sup.-3                                                                      102      46                                           92         3.5 × 10.sup.-3                                                                       81      32                                           93         5.0 × 10.sup.-3                                                                       68      30                                           ______________________________________                                    

From Table XIV it is apparent that a significant reduction in afterglowcan be realized when indium is incorporated in concentrations range fromw=1.0 ×10⁻⁶ to 5×10⁻³. At higher concentrations reductions in promptemission relative intensity are obJectionably large. When indiumconcentrations are in the range of from w=2×10⁻⁶ to 1×10⁻³, thephosphors exhibit both significant increases in prompt emission relativeintensity and significant reductions in afterglow.

Examples 94-99 Indium and Neodymium Additions

The purpose of presenting these investigations is to demonstrate thathost phosphors satisfying the requirements of this invention havingtheir intensity of prompt emission increased by titanium incorporationand their afterglow eliminated by the incorporation of neodymium exhibitlittle, if any, loss of prompt emission relative intensity by thefurther addition of indium. Indium was provided as described in Examples86 93 above. Aqueous solutions of neodymium were prepared from Nd(NO₃)₃6H₂ O (REacton, 99.99%, Rare Earth Products) and were used in Examples94 to 99 inclusive.

The relative luminescence outputs and

values of the indium and neodymium doped samples and the control lackingeither indium and neodymium addition are set out in Table XV.

                  TABLE XV                                                        ______________________________________                                        DTi.sub.x In.sub.w Nd.sub.v                                                                           Relative Relative                                     Example    Concentration                                                                              Intensity                                                                              Afterglow                                    ______________________________________                                        73c (Control)                                                                            0.00         100      100                                          94         w (5.0 × 10.sup.-5)                                                                  98       *                                                       v (5.0 × 10.sup.-6)                                          95         w (5.0 × 10.sup.-5)                                                                  95       *                                                       v (1.0 × 10.sup.-5)                                          96         w (1.5 × 10.sup.-4)                                                                  101      *                                                       v (5.0 × 10.sup.-6)                                          97         w (1.5 × 10.sup.-4)                                                                  96       *                                                       v (1.0 × 10.sup.-5)                                          98         w (2.5 × 10.sup.-4)                                                                  102      *                                                       v (5.0 × 10.sup.-6)                                          99         w (2.5 × 10.sup.-4)                                                                  96       *                                                       v (1.0 × 10.sup.-5)                                          ______________________________________                                         *Afterglow at or below detection limit with relative afterglow not            exceeding 1                                                              

From Table XV it is apparent that afterglow was eliminated without anyloss in the prompt emission intensity of the phosphor. By comparingTables XIII and XV it is apparent that the combination of indium andneodymium is superior to the use of neodymium alone in titaniumactivated hafnia host phosphors. Investigations substituting other rareearths for neodymium in hafnia host phosphors containing indium haveproduced results ranging from somewhat better to somewhat worse thanthat obtained using indium alone as a dopant for titanium activatedhafnia host phosphors. In no instance was afterglow eliminated by acombination of indium and other rare earth dopants.

Comparative Examples 100-105 Gallium Addition

The purpose of presenting these investigations is to demonstrate thathost phosphors satisfying the requirements of this invention havingtheir intensity of prompt emission increased by titanium incorporationdo not benefit to any significant degree by the incorporation ofgallium. Gallium ions were provided in the form of an aqueous solutionof gallium nitrate in Example 100 and as solid gallium nitrate inExamples 101-105.

The relative luminescence outputs and afterglow values of the neodymiumdoped samples and the control lacking rare earth addition are set out inTable XVI.

                  TABLE XVI                                                       ______________________________________                                        DTi.sub.x Ga.sub.w                                                                                    Relative Relative                                     Example    w            Intensity                                                                              Afterglow                                    ______________________________________                                         73d (Control)                                                                           0.00         100      100                                          100 (Comp. Ex.)                                                                          2.5 × 10.sup.-6                                                                      101      89                                           101 (Comp. Ex.)                                                                          5.0 × 10.sup.-5                                                                      100      96                                           102 (Comp. Ex.)                                                                          1.0 × 10.sup.-4                                                                       98      97                                           103 (Comp. Ex.)                                                                          2.5 × 10.sup.-4                                                                       97      111                                          104 (Comp. Ex.)                                                                          5.0 × 10.sup.-4                                                                      100      103                                          105 (Comp. Ex.)                                                                          1.0 × 10.sup.-3                                                                      102      98                                           ______________________________________                                    

From Table XVI it is apparent that gallium has little influence onprompt emission relative intensity and mixed effects on afterglow,ranging from reducing afterglow slightly at the lowest concentration tomarkedly increasing afterglow at a higher concentration.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

What is claimed is:
 1. A phosphor capable of absorbing X-radiation andemitting longer wavelength radiation comprised of monoclinic crystals ofa titanium activated hafnium dioxide and/or zirconium dioxide hostconsisting essentially of oxygen and combined elements satisfying therelationship

    DTi.sub.x M.sub.y In.sub.w

wherein D represents at least one of zirconium and hafnium ions; Mrepresents at least one alkali metal; w is in the range of from 1×10⁻⁶to 5×10⁻³ ; x is in the range of from 3×10⁻⁴ to 1.0; and y is up to 1;the phosphor exhibiting reduced afterglow when excited by X-radiation ascompared to the phosphor as defined above absent indium.
 2. A phosphorcapable of absorbing X-radiation and emitting longer wavelengthradiation comprised of monoclinic crystals of a titanium activatedhafnium dioxide and/or zirconium dioxide host consisting essentially ofoxygen and combined elements satisfying the relationship

    DTi.sub.x M.sub.y In.sub.w Nd.sub.v

wherein D represents at least one of zirconium and hafnium ions; Mrepresents at least one alkali metal; v is in the range of from 5×10⁻⁸up to 5×10⁻⁴ ; w is in the range of from 1×10⁻⁶ to 5×10⁻³ ; x is in therange of from 1×10⁻⁴ to 1.0; and y is up to 1; the phosphor exhibitingreduced afterglow when excited by X-radiation as compared to thephosphor as defined above absent neodymium.
 3. A phosphor according toclaim 1 or 2 in which D satisfied the relationship

    Hf.sub.1-z Zr.sub.z

in which z is in the range of from 4×10⁻⁴ to 0.3.
 4. A phosphoraccording to claim 3 in which z is in the range of from 1×10⁻³ to 0.2.5. A phosphor according to claim 4 in which z is in the range of from2×10⁻³ to 0.1.
 6. A phosphor according to claim 3 in which x is in therange of from 3×10⁻³ to 0.25.
 7. A phosphor according to claim 1 or 2 inwhich x is in the range of from 3×10⁻⁴ to 0.5.
 8. A phosphor accordingto claim 7 in which y is in the range of from 1×10⁻⁴ to 0.2.
 9. Aphosphor according to claim 8 in which y is in the range of from 8×10⁻⁴to 0.2 and the alkali metal ions include at least one of lithium,sodium, and potassium.
 10. A phosphor according to claim 1 or 2 in whichy is in the range of from 1×10⁻⁴ to
 1. 11. A phosphor according to claim1 or 2 in which w is in the range of from 2×10⁻⁶ to 1×10⁻³.
 12. Aphosphor according to claim 2 in which v is in the range of from 2×10⁻⁶to 1×10⁻⁵.